Recently, efforts has been made to develop the so-called active matrix type liquid crystal display panel which has a thin film transistor as a switching element for each picture element for obtaining a high quality picture. Compared with the simple matrix type, this type of liquid crystal display panel has high contrast ratio regardless of an increasing number of scanning lines and hence is being rapidly adopted for such fields as pictures for an engineering work station (EWS) which require high capacity and other fields where high picture brightness is required.
Widely used for liquid crystal display panels of the active matrix type is the TN (twisted nematic) system. This system has between two opposing substrates a liquid crystal panel of construction in which liquid crystalline molecules are twisted vertically 90.degree. by two polarizing plates. The TN system has two types. One type is the NW (normally white) mode in which the aforementioned two polarizing axes are perpendicular to each other, and the direction of the major axis of the liquid crystalline molecules in the interface of either substrate is parallel or perpendicular to the polarizing axis of the polarizing plate on the same side. The second type is the NB (normally black) mode in which the aforementioned polarizing axes are parallel to each other, and the direction of the liquid crystalline molecules is parallel or perpendicular to the polarizing axes of the polarizing plates on the same side.
With such a liquid crystal display panel a so-called rubbing treatment was neccessary. A rubbing treatment is a treatment in which a polymer thin film of e.g. polyimide is formed on a liquid crystal display panel's substrate and rubbed in a given direction with nylon, polyester cloth or the like, for equalizing the orientation direction of liquid crystalline molecules. As a result, it is possible to obtain a mono-domain all over the panel.
Since, however, this rubbing treatment possibly causes electrostatic destruction, dusting, staining of the film or the like, a process which does not include a rubbing treatment has been developed, for example, a process of orienting nematic liquid crystals by forming micro-groups over a substrate by application of the photolithograph (Kawada et al.: preliminaries for the 17th Forum on Liquid Crystals, 2F108).
With the widening scope of uses for liquid crystal display panels and also with efforts for up-sizing screens in full colors, however, the problem of the narrowness of visual angle, even in liquid display panels of the active matrix type with its high display quality has been studied.
In a liquid crystal display panel, liquid crystalline molecules try to orient in the direction of the electric field with release of their twisted construction when voltage is applied between the substrates, but the polarization (condition) of the light transmitting through the panel varies due to spatial orientation of the then liquid crystalline molecules, and the transmitted light is dimmed. In NW mode, for example, white display is obtained when no voltage is applied, and black display is obtained when a sufficiently high voltage is applied. Even if the liquid crystalline molecules are under the same spatial conditions when the voltage applied is constant, light's polarization conditions vary according to the incident direction of light into the liquid crystal layer. Hence, light transmission intensity is bound to vary when compared with that where incident light is perpendicular to the panel. Further, the transmitted light intensity is determined by the orientation of the liquid crystalline molecules located in the plane amid the substrates (mid plane). In other words, light intensity is determined by the inclination angle of the major axis of the liquid crystalline molecules in the mid-plane and the position from which the light transmitted through the liquid crystal panel is observed.
With both the aforementioned rubbing method and the photolithographic method, the direction along which the liquid crystalline molecules in the mid-plane are inclined by the electric field is fixed with respect to the panel plane. This gives rise to some difference in birefringence according to the visual angle (viewing angle), that is, a variation in the polarizing stage of the transmitted light and a problem relating to viewing angle. This results in large variation in contrast or hue according to the viewing angle.
In recent years, therefore, great efforts have been made to develop techniques for increasing the viewing angle in a liquid crystal display panel of the active matrix type. For example, a method of increasing the viewing angle by dividing each picture element of TN type liquid crystal display panel into two domains different in orientation condition has been proposed by, among others, T. Takatori et al.: JAPAN DISPLAY '92, PP. 591, (1992). This method, for example, consists of dividing a picture element into two parts so that in each picture element two kinds of liquid crystalline molecules inclined by electric field located in the mid-plane are created. This increases the viewing angle by mutually compensating the difference in birefringence determined by the viewing angle. This method is called the mutual compensation type whererin viewing angle compensation is made in two domains.
Since in the above methods the orientation domain of one picture element has to be divided into two, the exposure process has to be made anew or the rubbing treatment has to be made twice. The entire process is complicated.
A still more advanced method, which does not require a rubbing treatment and which is effective for a simultaneous increase in the viewing angle was proposed (by Y. Toko et al.: SID '93 DIGEST. PP. 622, (1993)). This method consisted of injecting a liquid crystal material between substrates coated to form an oriented polyamide film without a rubbing treatment at no less than the nematicisotropic phase transition temperature and then cooling. Thus, a plurality of domains different in orientation from liquid crystal molecules by random orientation of liquid crystalline molecules is formed which is an effective method for increasing the viewing angle.
This display method for the random-oriented liquid crystal display panel without a rubbing treatment will be described below with reference to FIG. 7.
FIG. 7 is a perspective view showing the random orientation of liquid crystal molecules without an electric field in the liquid crystal display panel. The top substrate 701 and the bottom substrate 702 a have pair of polyimide oriented films formed thereon and with a given gap between the substrates. Chiral nematic liquid crystal spontaneously having approx. 90.degree. of twist angle is inserted at a temperature above the nematic-isotropic phase transition point followed by cooling to room temperature. The directions of the liquid crystalline molecules then formed in the interface of the substrates in the individual domains are random at exactly equal probability, but in a given liquid crystal domain 706 a plurality of sub-domains are formed since there is a mutual twisting in construction of 90.degree. between the liquid crystalline molecules in the interface of the top and bottom substrates.
Although in this panel the liquid crystalline molecules 705 in the mid-plane are initially oriented substantially horizontal but are inclined progressively with minimization of dielectrically free energy (dielectric constant anisotropy&gt;0) under application of voltage and toward perpendicular orientation as the voltage applied gets higher. Since the liquid crystalline molecules 705 in the mid-plane are those located at the perpendicular center of the top and bottom substrates, they are located, when the angle of twist is 90.degree., at 1/2 of the angle between the liquid crystalline molecules 703 of the top substrate and the liquid crystalline molecules 704 of the bottom substrate, or 45.degree.. Since the viewing angle (direction) is determined by the voltage-induced direction of inclination of crystalline molecules 705 in this mid-plane, the viewing angle is constant in a given liquid crystal domain. Hence this direction is microscopically uniformized if a sufficient plurality of these liquid crystal domains having random orientations should exist, wherein the transmission intensities determined by observation from various directions become roughly symmetric with no dependency on the viewing angle.
As seen from the above explanation, however, if the number of liquid crystal domains in a single picture element should not be sufficient or if the direction of orientation of each liquid crystal domain should not be perfectly random, perfect compensation cannot be obtained and residual, dependency on the viewing angle is present. In the above described method, compensation is made by the presence of a plurality of liquid crystal domains 706 in a single picture element. Therefore the method is a multi-compensation type.
In this method compensation is made by the presence of a plurality of liquid crystal domains in a single picture element, hence the method may be called of the multi-compensation type.
The aforementioned method of increasing the viewing angle by means of random orientation, however, had the following problems.
(1) A problem that a special manufacturing apparatus is required for injection of a liquid crystal material at a temperature higher than the nematic isotropic phase transition temperature to ensure against the defect of flow orientation due to liquid crystalline flow. Another problem is lowering of the display quality and reliability if the normally practiced method of injecting liquid crystal in vacuum, in which the liquid crystal material is subjected to vacuum condition at a high temperature and a risk of loss of the low-boiling substances contained in the liquid crystal material.
(2) Liquid crystal display panels for miniature video display devices and miniature information terminals may have RGB's (Red, Green, Blue) electrode pitch of less than, for example, 50 .mu.m and in such minute picture element it is difficult to keep stable a large number of still more minute liquid crystal domains. Linear orientation flaws (discrimination lines) resulting from application of a voltage are unstable in terms of energy and tend to grow to be still larger liquid crystal domains by fusion with adjacent liquid crystal domains. This means a major orientation flaw affecting the display quality of a liquid crystal display panel.
Also, if the size of each liquid crystal domain should not be sufficiently small, there is a marked lowering of liquid crystal. For this reason, the domains have a large size, resulting a a difference of transmittivity between each domain. As a result, the problem with out diagonally adjacent liquid crystal domains (hereinafter called "optical roughness") arose.
(3) Further, the discrimination lines produced by application of an electric field were difficult to fade away during a black display by application of a high voltage, and the resulting of escape of light caused lowering of contrast.
(4) It was necessary to arrange the display so that the viewing angle characteristics of the polarizing element itself were optimum when the intersection angle of the polarizing element was 90.degree. (so-called "Cross-Nicol-State").